Telephone system



March l7 1942. B. sElFr-ERT TELEPHONE SYSTEM Filed May 51, 1940 PatentedMar. 17, 1942 TELEPHONEl SYSTEM Bernhard SeiHert, Berlin, Germany,assignor to Fides Gesellschaft fr die Verwaltung und Verwertung vongewerblichen Schutzrechten mit beschrnkter Haftung, Berlin, Germany, acorporation of Germany Application May 31, 1940, Serial No. 337,997 InGermany MayZ, 1939 4 claims. `(o1. 179-86) The invention relates to acircuit Varrangement for party line telephone systems, more particularlytrain control lines, in which the branch stations are called selectivelyfrom a main exchange or station.

In known arrangements of this kind the branch stations are selected fromthe main station transmission of impulse trains which actuate thestep-by-step switching mechanism in all the branch stations, but putsthe call through to the desired branch station only. This method ofselective calling requires selectors of large capacity if, asis usual, alarge number of stations are connected to the line, and it furtherrequires a relatively large source of current in order to ensure theoperation of the selectors. This arrangement is unsatisfactory for thefurther` period necessary for a call to be completed is,

in the known arrangements, relatively long; during this interval thestations are disconnected from the line, so that during this time nomessages, not even urgent messages, can be transmitted to the mainstation.

Arrangements for the selective call of .branch stations are alreadyknown which dispense with the use of selectors at the stations in whichdifferent frequencies to which the receiving relays in the subscribersstations are tuned are used. These arrangements require that a largenumber of frequencies be provided. Other known arrangements which firstmark a group of stations over the one or the other conductor of theparty line by connecting battery potential and subsequently effect theindividual selection by a transmission of different frequencies resultin a reduction of the number of frequencies, but are not suitable for alarge number of stations, moreover, these arrangements require theconnection of earth potential at the station. The reverse arrangement,to effect at first group selection by means of predetermined frequenciesand to effect the individual selection by means of direct current doesnot permit the increase of the number of subscribers beyond a certainlimit, unless a large numbers of frequencies are again to be used. Thesearrangements also require the connection of earth potential at thestation.

Furthermore it is valready known, to effect selective calling ofstations by means of high battery-potentials in combination withdifferent current directions and the use of separate circuits over theone or other conductor of the party conductors to earth. But also thisarrangement can only be used for a limited number of stations, unlessvery large batteries are 'provided at the main station. Here again earthpotential must be provided at the stations.

The present invention has the advantage over these known arrangements,that it permits the connection of a large number of branch stations' andyet requires the provision of a small number of selecting frequencies inthe exchange station anda small number of relays `at the partylinestations. Moreover, it doesnot necessitate the provision of earth at thestations; this is of importance, if repeaters are to be used. Thisobject of the invention is achieved in that a plurality of frequencies(for example two) are operative at the branch stations upon a number ofreceiving relays corresponding to the number of frequencies provided inVoltagel combinations corresponding with the desired branch station.

The drawing illustrates, by way of example,an embodiment of theinvention. In Fig. 1 a party line GL is shown, comprising the mainexchange or station A and a number of branch stations, of which only thestations TI, T2, and T3 are shown. The main station is provided with twosources of alternating current WI and W2, WI having a frequency of 15Hz. and W2 a frequency of 100 Hz. The transformers Trl and Tr2 aretapped at various points so that different voltages may be tapped off.By the depression of the key provided upon the key board T for thedesired stations an impulse consisting of the frequency-mixture of 15HZ. and 100 Hz. of the requisite voltage combination as set out in Fig.2 is supplied to the party line. Thus, for example, the station Tl iscalled by a frequency of 15 Hz. with a voltage of 10 v. and a frequencyof 100 Hz. with a voltage of 100 v.; the station T2 is called by afre-quency of 15 Hz. with a voltage of 20 v. and a frequency of 100 Hz.with a voltage of v.; the station T3 is called by a frequency mixture of15/100 Hz. and a voltage combination of 30/80 volt. The

voltage combinations for calling the other stations are shown in Fig. 2.

The branch stations which are connected to the party line overtransformers UI, U2, U3, etc. are each provided with two alternatingcurrent receiving relays RI, R2, of which the relay RA is responsive tol5 I-IZ. and the relay R2 to 100 Hz. In view of the voltage drop uponthe line these relays are associated in each station with seriesresistances or potentiometers, not shown in the drawing in order toadjust the diii'erent voltages to the values of the respective stations.If, therefore, for example, the main station calls the station T3 bymeans of the key T3, the frequency 15 Hz. with a voltage of 30 V. andthe frequency 100 HZ. with a voltage of 80 v. are received in thestation T3. The two relays Rl, and R2 of this station operate andconnect the alarm WK at this station over the contacts rl and r2. Attheother stations one relay only is operated by the frequency mixtureapplied to the liney during-the call. In the stations Tl and; T2r onlythe relays RI are energised which are adjustedV to a-n operating voltageof and 20 v., respectively. The relays R2 in these two stations cannotoperate, as they are energised only by 100 and 90 v., respectively. Inthe branch stations T4 to TIO which are connected subsequent to thestation T3 all the relays R2 operate, as these relays are adjusted tooperating voltages below 80. v. The relays R'I, however, cannot operatein these stations, as these relays require a voltage of more than 30 v.for their operation.

By an increase of the voltagev or ofy the number of frequencies, or byboth means jointly the number of combinations and hence the number ofstations which it is possible to connect to the party line may beincreased to any convenient limit.

I claim:

1. In a selective calling system,` a line having a plurality ,ofstations thereon, a plurality ofl sources of alternating current ofdifferent frequency, and means for connecting said sources to said linein different voltage combinations one for each station, and relays ateach station responsive to said frequencies of current at a particularvoltage.

2. In a selective calling system having a plurality of stationsconnected to a line, means for connecting alternating current of twodifferent frequencies to the line over different connections to transmitcurrent of different voltages, a relay at each of the stationsresponsive to one frequency but to diierent voltages thereof, andanother relay at each station responsive to the other frequency but todierent voltages thereof.

3. A system such as claimed in claim 2 wherein the relays at thestations along the line in one direction are responsive to progressivelyincreasing voltages of one frequency while the other relays at thestations along the line in the same direction are responsive toprogressively decreasing voltagesof the other frequency.

4; In a` selective calling system, a line having a plurality of stationsthereon, a plurality of relays at each station, each relay responsive toalternating current of a certain frequency and only above a certainvoltage, a plurality of sources of alternating current, means forconnecting said sources to the line via certain voltage taps, saidsources when connected to the line via certain taps energizing all ofthe relays at only one. of said stations to thereby signal only thatstation.

BERNHARD SEIFFERT.

